The present invention relates generally to a method and apparatus for measuring puncture resistance in pressurized ultra-thin aluminum containers, and more particularly puncture resistance in aluminum containers for carbonated beverages.
The invention relates to a method and apparatus for measuring resistance in pressurized aluminum containers. It is specifically related to an improved method and apparatus for measuring puncture resistance in aluminum cans containing carbonated beverages for the purpose of assessing the effect of engineering changes to the can on its ability to resist puncture and rupture failures.
The carbonated beverage industry, and particularly the soda and beer industries are familiar with the use of aluminum containers or cans for packaging of carbonated beverages. Aluminum cans are used primarily as containers for retail sale of beverages in individual portions. Annual sales of such cans are in the billions and consequently, over the years, their design has been refined to reduce cost and improve performance. Other refinements have been made for ecological purposes, to improve reclamation and promote recycling.
Aluminum cans are usually formed from thin sheets of aluminum alloy, such as the aluminum alloy 3004 or 3104. The interior of the can is spray coated after forming. The coating protects the aluminum in the interior of the can from the corrosive beverage products that the can is designed to hold, and conversely protects the beverage flavor from being altered due to contact with the aluminum can. The starting gauge of the aluminum coil is approximately 10.5 mils (0.0105 inches) thick before any can forming begins. The coil is blanked and cold worked to form a can body, consisting of a bottom portion and a generally cylindrical portion typically referred to as the sidewall, which are joined to a separately manufactured can lid or closure. The can forming process and general can design and dimensions are well known in the art and will not be discussed in detail. Each component of the can has certain specifications and requirements, which are also well known in the art. For instance, the upper surface of the can lids must be configured to nest with the lower surface of the can bottoms so that the cans can be easily stacked one on top of the other. Additionally, the cans must be capable of withstanding pressures of approximately 90 psi, once filled with carbonated beverage.
The aluminum can industry continues to develop new ways to improve can performance and reduce costs without sacrificing the ability to satisfy these functional requirements. Significant cost reductions in manufacturing aluminum cans may be realized in material savings, primarily through the use of thinner metal to form the cans. Optimization of can and lid geometry has permitted continued use of thinner metal materials without seriously impacting the ability to satisfactorily maintain functionality of the cans.
Although the use of thinner metal has its advantages and can be used while still meeting essential functional requirements, it also has its drawbacks. One of the drawbacks relates to decreased puncture and fracture resistance in the aluminum cans manufactured with thinner metal. The current typical manufacturing process for aluminum beverage cans optimizes the metal usage by drawing and ironing the sidewall of the can. This results in the can being thinner than the coil starting gauge in the can sidewall, known as the thinwall portion, which is approximately 0.004xe2x80x3 thick. Thinner sidewalls in aluminum cans are more susceptible to being punctured by contact with an external surface and are more susceptible to fracture failure. The principles underlying failures of pressure vessels due to puncture and fracture are well known in the art and do not require further detailed discussion. Relevant principles related to puncture and fracture that are specific to aluminum beverage cans are discussed below.
Failures in aluminum beverage cans manifest as either a rupture or a leak, depending on various conditions. A fracture in an aluminum can results in failure of the can and may be caused by slow forces, such as fatigue or corrosion, or by a puncturing event. A puncturing event is generally a dynamic contact between a foreign, external surface and the container. Fracture resistance is generally considered a material property that quantifies a material""s resistance to crack growth, while puncture resistance is a container property that quantifies the load, deflection and/or energy that a particular container is capable of withstanding before fracturing due to an impact with an external surface. A can fracture that grows rapidly is known as a rupture, where the can splits open and the contents, which are under pressure, are violently discharged. A fracture that does not result in a rupture is known as a leak, where the pressurized contents slowly leak out of the can. The tendency of a pressure vessel, such as a can, to rupture or leak in response to a fracture is described by the fracture mechanics design theory known as Leak-Before-Break.
Rupture failures in industrial pressure vessels are extremely dangerous and can cause death, injury and substantial damage. By comparison, damage caused by the rupture of aluminum beverage cans is less severe. The contents of the can, and in some rare occasions small pieces of the container itself, are projected into the local surroundings. Regardless of the type of pressure vessel, a leak type of failure is preferable to a rupture if a failure occurs.
Although the aluminum can industry strives to avoid failures altogether, some events of can failure are inevitable. Leak-Before-Break theory gives canmakers the ability to predict the response of cans whose thinwalls have been fractured. The leak versus rupture response of the can is governed by the fracture resistance, which is a property of the material, the failure hoop stress (controlled by the internal pressure and the wall thickness), and the length of the flaw that causes the fracture. In addition to fracture resistance, the puncture resistance of a particular container is important in determining whether particular containers or container designs are more or less susceptible to a failure by puncture than other containers.
Instrumented impact testing techniques have previously been used to measure puncture resistance in various containers, including pressurized aluminum containers. Instrumented impact testing is similar to conventional impact testing with the exception that in an instrumented test the force applied to the specimen, in this case the aluminum can, is measured continuously throughout the test. Two basic types of instrumented impact testing technology are known in the prior art, pendulum or drop-weight testing that use gravity to impact the specimen and servo-hydraulic or pneumatic machines that force an impactor through the specimen. The design of many of these prior art systems, which impact the side of the can from above, causes the measurements to be taken in an area of the can backed by a gas bubble, rather than a liquid. Impact in the area of a gas bubble produces results atypical of the most common conditions of external impact for end-uses of the can.
The present invention is related to an improved method and apparatus for measuring puncture resistance in aluminum cans. The puncture resistance gauge of the present invention is an improved impact-testing device that more accurately simulates typical end-use conditions of a filled aluminum can undergoing a dynamic impact with an external object. The improved puncture gauge and method of use according to the invention measures various puncture characteristics or parameters of an aluminum can during an impact event, including the force that the aluminum can is capable of absorbing before puncturing, and the deflection that the can withstands in absorbing this applied force.
The present invention provides an improved method and apparatus for measuring puncture resistance in thin aluminum beverage cans. A preferred embodiment of the disclosed puncture gauge uses a horizontal gauge configuration such that the filled sample can is upright during testing. As described below, the use of an upright sample can where the area of impact is backed by liquid more accurately simulates typical end-use conditions. A preferred embodiment of the horizontal puncture gauge also includes a carriage to removably hold the penetrator in place as it is driven down the length of the puncture gauge, preferably along a set of tracks for stability and to provide a straight path, toward the sample can.
Another feature of a preferred embodiment of the puncture gauge of the present invention is a splash chamber for containing the sample can during testing. The use of a splash chamber protects sensitive electronic measuring equipment from the spray of liquid if the impact results in a through-wall puncture. A final feature of a preferred embodiment of the invention includes a data acquisition system to collect, convert, analyze, and report test data measured from various sensors during testing. These features, described in detail below, may be used singularly or combined to test cans of various geometries under development for puncture resistance. Aluminum cans with varying sidewall thicknesses may be tested during development in an effort to achieve even thinner aluminum cans that still meet minimum threshold requirements for resisting puncture failure. Additionally, these features may be used singularly or in combination for the testing of aluminum can production batches, likewise to ensure that manufactured cans are meeting industry puncture resistance tolerances. The method of using the horizontal puncture gauge of the invention is also described below.